They are normal humans infused with mako to give them super strength and agility. The mako also gives members of SOLDIER a unique trait - glowing blue/green eyes.

The group was originally led by Lazard Deusericus in Crisis Core, and is divided into three classes: 1sts, 2nds, and 3rds, with 1sts being the best. The classes are separated by their uniform colors. 1sts wear black uniforms, 2nds wear purple, and 3rds wear blue. 1st Class SOLDIERS, it would seem, have permission to add to and subtract from the standard uniform.